A useful technology for recovering usable base stock from used lube oil can employ nano-filtration membranes. Colloquially, a process based upon use of open osmosis membranes can be referred to as “nano-filtration”. However use of such membranes is distinguishable from “filtration” in the following respects: separation of fluids takes place at the membrane surface based on attractions and repulsions of specific dissolved chemical moieties; this is not a filtration of solid particles in the traditional sense. This is instead analogous to reverse osmosis.
Accordingly, although the expressions “nano-filtration”, “micro-filtration”, “ultra-filtration”, “hyper-filtration”, “filtrate”, “permeate”, “filtering medium” may be used in the course of this disclosure, these expressions are actually intended to extend to the case where there is a separation of a liquid stream into a permeate and a concentrate by any analogous process. The invention is not limited to the use of a specific type of membrane.
Lubricating (lube) oils consist of a starting base stock and an additive package. The inherent value of lube oil has led to many attempts at reclaiming the base stock from used lubricating oil with varying levels of success. One technique is to pass the used oil, appropriately pre-conditioned, over a nano-filtration membrane.
Attempts at using commercially available membrane containment systems include the DDS (De Danske Sukkerfabrikker) plate and frame equipment described in U.S. Pat. No. 3,872,015.
A previous patent to Kutowy et. al. U.S. Pat. No. 4,814,088 of Mar. 21, 1989 addresses a membrane-based ultrafiltration process to clean mildly used lube oil as well as crude oil and other chemicals. The contents of this and the following Kutowy US patents are adopted herein by reference.
Other patents to Kutowy et. al., U.S. Pat. No. 5,002,667 Mar. 26, 1991, and U.S. Pat. No. 5,624,555 Apr. 29, 1997 describe using a metallic plate and frame for membrane support. In particular the latter patent describes a paired-membrane panel assembly which incorporates two membranes each overlying a respective permeable, e.g. perforated, membrane support panel located adjacent to the individual membrane's permeate or low pressure side. Such paired membrane support panels are mounted in parallel exposing parallel membranes to feedstock flowing in the same direction.
Feedstock in a membrane system usually requires some pre-treatment. Used lube oil becomes unfit for its purpose due to physical contamination and chemical changes. Water and glycol exist in several forms in used crankcase oil. It is desirable for such contaminants to be reduced to a minimum before a feedstock is exposed to a nano-filtration membrane.
The presence of water and glycol in particular poses a problem to base stock reclamation through small pored membranes such as nano-filtration membranes. This is because of the formation of emulsions that tend to stick and block pores in membranes. Water and glycol have to be virtually completely removed for a nano-filtration membrane-based process to be most effective. Thus the feedstock for a nano-membrane filter should be “membrane compatible” and “feedstock” as used herein is so intended.
Use of nano-membrane filters gives rise to a number of structural requirements for the membrane support structure.
In order to provide a useful quantity of permeate when exposing liquid feedstock to a membrane, the membrane is normally supported to carry a substantial trans-membrane pressure, e.g. on the order of 100 psig. Further, passing a flow of feedstock as a working fluid over a membrane surface under pressure is preferably done in a confined space, e.g., a depth that is preferably only a moderate multiple of the thickness of the membrane and/or the membrane and its supporting perforated panel. This confined space has a preferred depth to maximize the quantity of working fluid that comes into contact with the membrane surface and to maintain flow velocity. (“Fluid” as used herein refers to a liquid unless the context indicates otherwise.) Establishing the correct flow rate over a membrane helps keep the membrane surface clean.
As a consequence of this narrow confinement the working fluid will suffer a pressure drop as it passes as a cross-flow along the length of a membrane. Over a distance of, say, 2 meters in length, the pressure drop could be on order of 10 psig for used lubricating oil, depending on the depth and viscosity of the flowing feedstock layer.
If the working fluid is to be exposed to an extended surface area of membrane, e.g., past multiple supported membrane surfaces connected in series, this cross-flow pressure loss will accumulate. All along the membrane surfaces the pressure must be kept above a minimum pressure, for example 100 psig, to sustain effective permeation. Therefore the entry pressure of the working fluid as it is exposed to the first membrane must, according to one solution, be high enough to accommodate the subsequent pressure losses for the flowing working fluid to maintain the minimum, e.g. 100 psig, pressure needed to force permeate through the membrane at a reasonable rate.
To contain high pressure fluid requires strong frames, sealing plates and seals. Typically these are made of steel. As the requirement for strength goes up (to accommodate higher pressures) the weight of such supporting assemblies increases. This places higher demands on the handling apparatus as well as imposing increased cost.
It would therefore be desirable to provide a support assembly for filter membranes having minimized weight and strength requirements. Correspondingly, the input pressure of the working fluid should be limited to the extent practically possible. This invention addresses such objectives.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.